The present invention relates to an ink-jet recording head adapted to discharge minute ink droplets from a nozzle to record characters or images, and an ink-jet recording apparatus in which the ink-jet recording head is installed.
Hitherto, as one of this type of recording heads, an xe2x80x9con-demand type ink-jet recording headxe2x80x9d that discharges ink droplets from a nozzle according to printing information has been extensively known, An on-demand type ink-jet recording head has been disclosed in, for example, Japanese Examined Patent Publication (JP-B) No. 53-12138. FIG. 11 is a sectional view that conceptually shows a basic construction of an ink-jet recording head known as a Caesar type among the on-demand type ink-jet recording heads.
As shown in FIG. 11, in the Caesar type recording head, a pressure generating chamber 91 and a common ink chamber 92 are coupled via an ink supply aperture (ink supply passage) 93 at an ink upstream side. At an ink downstream side, the pressure generating chamber 91 and a nozzle 94 are coupled. A bottom plate of the pressure generating chamber 91 shown in the drawing is composed of a diaphragm 95, and a piezoelectric actuator 96 is provided on the rear surface of the diaphragm 95.
In such a construction, to perform a printing operation, the piezoelectric actuator 96 is driven to displace the diaphragm 95 on the basis of printing information, thereby suddenly changing the volume of the pressure generating chamber 91 to produce a pressure wave in the pressure generating chamber 91. The pressure wave causes a part of the ink charged in the pressure generating chamber 91 to be injected outside through the nozzle 94 in the form of an ink droplet 97. The discharged ink droplet 98 impacts onto a recording medium, such as recording paper, and forms a recording dot. Such a recording dot is repeatedly formed on the basis of the printing information thereby to record a character or an image on the recording medium.
Referring now to FIGS. 12(a) through (d) and FIG. 13, the relativity between the behaviors of a meniscus and printing performance will be discussed.
FIGS. 12(a) through (d) are sectional views illustrating a changing process of a meniscus M of the nozzle 94 in the aforesaid ink droplet discharging process, and FIG. 13 is a graph showing time-dependent changes of the position of the meniscus M after the ink droplet is discharged. Before the ink droplet 97 is discharged, the meniscus M is set so that it is positioned substantially flush with an aperture surface of the nozzle 94, as shown in FIG. 12(a). When the piezoelectric actuator 96 is driven and the ink droplet 97 is discharged, the meniscus M moves back into the nozzle 94 according to the amount of the discharged ink, as shown in FIG. 12(b). At this time, if the next discharge is implemented while the meniscus M is still back in the nozzle 94, as shown in FIG. 12(c), then a discharging condition (a droplet diameter, droplet speed, etc.) changes, or a discharge failure results. Hence, in order to achieve stable continuous discharge, it is important to wait until the meniscus M that has retreated is moved back to the vicinity of its initial position by the action of surface tension, as illustrated in FIG. 12(d), before the next discharge cycle is implemented. More specifically, it is crucial to start the next discharge cycle after a time required for refilling after the ink is discharge has elapsed (refilling time tr), as shown in FIG. 13.
From the descriptions above, it can be understood that a maximum discharging frequency fe of the ink-jet recording head depends on the refilling time tr of the head. More specifically, to attain high-speed recording by operating at the maximum discharging frequency fe, it is necessary to shorten the refilling time tr so as to satisfy a condition indicated by tr less than 1/fe. To be more specific, the refilling time tr can be reduced by increasing a cross-sectional area of the passage system formed of the nozzle 94, the pressure generating chamber 93, and the ink supply aperture (ink supply passage) 91, or by decreasing the viscosity of the ink thereby to decrease a passage resistance.
However, reducing the passage resistance results in a side effect of an increase in an overshoot Xmax of the meniscus M, as shown in FIG. 13, although the refilling time tr is shortened. More specifically, if the overshoot Xmax is large, then the condition (position or speed) of the meniscus M immediately before the discharge of the ink droplet 97 does not remain constant, leading to an inconvenient problem in that the droplet diameter or the droplet speed (discharging speed) of the droplet 97 varies. Therefore, to secure the accuracy in the droplet diameter or the droplet speed, it is required to control the overshoot Xmax of the meniscus M to a predetermined value or less. Especially to accomplish recording with high image quality by droplet diameter modulation, high accuracy is required of the droplet diameter and the droplet speed. For this reason, the overshoot amount Xmax must be approximately 10 xcexcm at maximum. A specific measure for suppressing the overshoot Xmax, the cross-sectional area of the passage system may be reduced or the ink viscosity may be increased so as to increase the passage resistance. As mentioned above, however, increasing the passage resistance causes the refilling time tr to be prolonged, so that high-speed recording is inconveniently sacrificed.
Thus, in the ink-jet recording head, it is extremely difficult to realize the recording with high image quality performed by droplet diameter modulation, and also high-speed recording at the same time, because the conflicting conditions, namely, the shortened refilling time tr and the restrained overshoot Xmax must be satisfied. In the past, however, attempts have been made to realize both the recording with high image quality and high-speed recording by maximizing the reduction in the refilling time and the restraint of the overshoot by devising the shapes of the nozzle or ink supply aperture (the ink supply passage) or the like, and by adjusting the viscosity of the ink.
According to the conventional attempts mentioned above, however, it has been extremely difficult to always achieve the shortened refilling time and the restrained overshoot over a wide operating temperature range of the apparatus. This is because the physical properties of the ink change due to environmental temperatures, and as a result, refilling characteristics markedly change.
As it will be discussed hereinafter, the refilling characteristics of the ink-jet recording head are governed by the inertance (acoustic mass) and the acoustic resistance of the passage system formed of a nozzle, an ink supply aperture (an ink supply passage), a pressure generating chamber, etc., and the acoustic capacitance of a meniscus. Among these factors, the inertance depends on the density of ink, the acoustic resistance depends on the viscosity of ink, and the acoustic capacitance depends on the surface tension of ink. Therefore, if the ink properties (density, viscosity, and surface tension) change according to environmental temperatures, then the characteristic parameters (inertance, acoustic resistance, and acoustic capacitance) of a passage system change accordingly, resulting in a significant change in the refilling characteristics. Actually, when the operating temperature range of the apparatus is 10 to 35xc2x0 C. (in the vicinity of room temperature), the dependence-on-temperature of the density and the surface tension can be almost ignored, but the temperature-dependent change of the ink viscosity cannot be ignored.
For instance, if the operating temperature of the apparatus is set to 10 to 35xc2x0 C., then the ink viscosity of a typical water-based ink develops an approximately 2.0-fold to 2.5-fold change. If the environmental temperature is low, then the ink viscosity increases with a resultant increase in the acoustic resistance of the passage system, making it difficult to obtain a desired refilling time tr. Conversely, if the environmental temperature rises, then the ink viscosity decreases, so that the overshoot Xmax of the meniscus increases although the refilling time tr shortens.
As a specific example, an example of a result of an experiment on an ink-jet recording head will be described. At room temperature (20xc2x0 C.), the refilling time tr was 90 xcexcs, and the overshoot Xmax r, was 5 xcexcm. In the ink-jet recording head, a target drive frequency is 10 kHz, and the allowable value of the overshoot Xmax is 10 xcexcm at this time. Hence, at the room temperature (20xc2x0 C.), the target value (100 xcexcs or less) of the refilling time tr can be secured, and the overshoot Xmax can be restrained. However, when the environmental temperature was lowered to 10xc2x0 C., then the overshoot Xmax was decreased to 2 xcexcm and therefore satisfied the overshoot condition, whereas the refilling time increased to tr 116 xcexcs, so that it was no longer possible to secure the target refilling time tr. Conversely, when the environmental temperature was increased to 35xc2x0 C., the refilling time tr was shortened to 72 xcexcs and therefore satisfied the refilling time condition, whereas the overshoot increased to 14 xcexcm, indicating that it was no longer possible to restrain the overshoot Xmax.
As described in detail above, since the ink viscosity greatly depends on temperature, it is extremely difficult to secure a target refilling time and to restrain the overshoot at the same time over a wide apparatus operating temperature range. Especially when the diameter of ink droplets to be discharged is set to a larger value so as to realize high-speed recording, marked deterioration is observed in the printing performance attributable to the temperature-dependent changes in the physical properties of ink. For example, when the recording resolution is set to a low value, approximately 400 dpi, the required ink droplet diameter (maximum droplet diameter) will be about 38 xcexcm to about 43 xcexcm. When such a large ink droplet is discharged, the amount of recession of a meniscus immediately after the discharge is large. This is likely to cause an increase in the refilling time or the overshoot, and also leads to increased susceptibility to the influences of the changes in environmental temperature. In fact, no ink-jet recording head has conventionally been available that is capable of perfectly securing the refilling time and restraining the overshoot at the same time under a condition where an ink droplet diameter of a maximum droplet diameter Hz or more, an overshoot allowable value of 10 xcexcm, and the apparatus operating temperature ranges from 10 to 35xc2x0 C. In the present specification, the droplet diameter means the diameter obtained by converting the total amount of ink discharged in one discharge cycle into a single spherical ink droplet.
Accordingly, an object of the present invention is to provide an ink-jet recording head capable of always securing a target refilling time and restraining overshoot at the same time even if an environmental temperature changes while an apparatus is in operation, and also capable of discharging at high speed a stable ink droplet with highly accurate droplet diameter and droplet speed. It is another object of the invention to provide an ink-jet recording apparatus in which the aforesaid head is installed.
To this end, the invention described in claim 1 relates to an ink-jet recording head that includes a pressure generating chamber filled with ink, pressure generating means for generating a pressure in the pressure generating chamber, an ink supply chamber for supplying the ink to the pressure generating chamber, an ink supply passage for establishing communication between the ink supply chamber and the pressure generating chamber, and a nozzle in communication with the pressure generating chamber, the pressure generating means causing a pressure change to take place in the pressure generating chamber so as to discharge an ink droplet from the nozzle, wherein the configurations of the nozzle, the ink supply passage, and the pressure generating chamber are set so that a total sum mT of the inertance and a total sum rT of acoustic resistance (the values at a temperature of about 20xc2x0 C.) of the nozzle, the ink supply passage, and the pressure generating chamber in an ink-filled state satisfy expressions (4) and (5):
0 less than mT less than 1.9xc3x97108[kg/m4]xe2x80x83xe2x80x83(4)
4.0xc3x971012 less than rT less than 11.0xc3x971012[Ns/m5]xe2x80x83xe2x80x83(2)
The invention described in claim 2 relates to the ink-jet recording head described in claim 1, wherein the nozzle has a tapered portion whose diameter gradually increases toward the pressure generating chamber, and the tapering angle of the tapered portion is 10 to 45 degrees.
The invention described in claim 3 relates to the ink-jet recording head described in claim 1, wherein the nozzle is composed of a straight portion provided in the vicinity of an opening and a tapered portion that gradually increases toward the pressure generating chamber, and the tapering angle of the tapered portion is 15 to 45 degrees.
The invention described in claim 4 relates to the ink-jet recording head described in claim 1, wherein the diameter of the nozzle gradually increases toward the pressure generating chamber, the longitudinal section of the nozzle is shaped into a curve that has a radius substantially equal to the length of the nozzle, and the length of the nozzle is 50 to 100 xcexcm.
The invention described in claim 5 relates to the ink-jet recording head described in claim 1, 2, 3, or 4, wherein the opening diameter of the nozzle is 25 to 32 xcexcm.
The invention described in claim 6 relates to the ink-jet recording head described in claim 1, wherein the ink supply passage is an ink supply aperture for establishing communication between the ink supply chamber and the pressure generating chamber.
The invention described in claim 7 relates to the ink-jet recording head described in claim 1, wherein the maximum droplet diameter of the ink droplet is set to 38 to 43 xcexcm.
The invention described in claim 8 relates to the ink-jet recording head described in claim 1, wherein the ink-jet recording head employs an ink with its surface tension set to 25 to 35 mN/m.
The invention described in claim 9 relates to the ink-jet recording head described in claim 1, wherein the ink-jet recording head employs an ink having its viscosity set such that the total sum rT of the acoustic resistance (the value at a temperature of substantially 20xc2x0 C.) of the nozzle, the ink supply passage, and the pressure generating chamber in an ink-filled state satisfies expression (6):
4.0xc3x971012 less than rT less than 11.0xc3x971012[Ns/m5]xe2x80x83xe2x80x83(6)
The invention described in claim 10 relates to an ink-jet recording apparatus incorporating the ink-jet recording head described in any one of claims 1 to 9.